Synthesis and Texture of Mesophase Templated Silica Layers Seeded with Nanoparticles
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Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
orient mesophase templated silica networks. Based on these previous results our approach to improve the texture consists in the introduction of nanoparticles in the gelling solutions. As a matter of fact this seeding could promote a heterogeneous nucleation of the templating mesophase and so the tailoring of the size of the ordered domains. Moreover magnetic nanoparticles could be used to orient the mesophases under magnetic field. This paper describes the experimental study which enabled us to introduce the nanoparticles in the complex synthesis solutions and the first results concerning the textural effects of the nanoparticle seeding. EXPERIMENT Materials The silica precursor was tetramethoxysilane (TMOS). Two cationic surfactants of alkyl trimethyl ammonium bromide type (CxH 2x+1(CH 3)3N+, Br-) were used: S8 (x=8) for the synthesis of the silica gels seeded with magnetic nanoparticles and S10 (x= 10) for the study of the surfactant-nanoparticle interactions. As a matter of fact, data on the critical micellar concentration (CMC) and on the molecular area (aO) are available in the literature for this last surfactant [14]. The other reagents were HCI or NH 3 aqueous solutions used to modify the pH of the various dispersions and to hydrolyze the silicon alkoxide, methanol and colloidal hydrosols with a volume fraction of oxide equal to 1%. The characteristics of the maghemite (y Fe2 0 3) particles used as ferrofluid source and of the silica nanoparticles used as reference non magnetic nanoparticles are given in table I. The compositions of the gelling solutions seeded with magnetic nanoparticles and the order of addition of the various reagents are reported in table II. The seeding level in the resulting gels corresponds to 1 particle per 105 nm3. Thick layers (1 mm
2 cmc
50-
0 0,00
0,04
[S10] (moV
249
0,08
b)
..
40
E :
pH =3 ;[TMOS] =1.23
--
20_•
0 0 ' 5 [S1O]
0
(moVl 1
1,5
Figure 1: Evolution of the size of the silica particles versus surfactant concentration in diluted aqueous solutions. a) effect of the pH; [TMOS]=0; b) pH=3; [TMOS]=l.23moI.1-l. The magnetic y Fe 20 3 particles are stable in a very limited range of pH around pH=2. At this pH they are positively charged and do not interact with the cationic surfactant. Unfortunately acidic pH does not favor the condensation and the polymerization of the silica network. The gelation times are longer than one month and the resulting gels do not retain their ordered structure during the stages of drying and thermal treatment, even after long aging periods before drying. In order to promote the polymerization by increase of the pH of the solution and to avoid particle aggregation, the silica precursor was introduced before the addition of the basic solution. The figure 2.a shows that, in these synthesis conditions, the zeta potential of the particles decreases and becomes negative at pH higher than pH=3. The particle size remains simultaneously constant except for pH=3 where
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